1. Field of the Invention
The present invention relates to a method for correcting defective pixels in a display device having a self-light emitting element. In particular, the invention relates to a method for correcting defective pixels in an active matrix display device having an EL (Electroluminescence) element. The invention also relates to a display device having a structure capable of easily correcting defective pixels.
2. Description of the Related Art
In recent years, a light emitting device having a light emitting element typified by an EL element has been actively developed, and it is expected to be widely used by taking advantages of the self-luminous type such as high image quality, wide viewing angle, thin thickness, and lightweight. As a light emitting element used for such a self-light emitting display device, an organic light emitting diode (OLED) (also called an organic EL element), an electroluminescence (EL) element, and the like have been drawing attention and used for an EL display and the like. Since the light emitting element such as an OLED is a self-luminous type, it has advantages such as higher visibility of pixels than that in a liquid crystal display, and fast response without requiring a backlight. The luminance of a light emitting element is controlled by a current value flowing through the light emitting element.
An EL element has an electroluminescent layer (layer containing an organic compound) between a cathode and an anode. The electroluminescent layer (layer containing an organic compound) may be formed of a single layer (only a light emitting layer) or a plurality of stacked layers. If the electroluminescent layer is formed of a plurality of layers, the layers may be stacked in any of the following orders from a semiconductor element side (pixel electrode side): (1) an anode, a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and a cathode; (2) an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode; (3) an anode, a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injection layer, and a cathode; (4) an anode, a hole injection layer, a hole transporting layer, a light emitting layer, a hole blocking layer, an electron transporting layer, and a cathode; (5) an anode, a hole injection layer, a hole transporting layer, a light emitting layer, a hole blocking layer, an electron transporting layer, an electron injection layer, and a cathode. Such a structure is called a forward staggered structure, and a pixel electrode functions as an anode. On the other hand, a structure where a cathode is formed first on the semiconductor element side (pixel electrode side) is called an inversely staggered structure, and a pixel electrode functions as a cathode.
An electroluminescent layer is required to emit light and flow current. Current flowing through an electroluminescent layer decreases inversely proportional to the power 3 of the film thickness; therefore, it is necessary to considerably reduce the thickness of the electroluminescent layer.
When the thickness of an electroluminescent layer is considerably reduced, however, the following problems occur: a short circuit between an anode and a cathode (hereinafter referred to as a short circuit between two electrodes) easily occurs. This is caused by the thin thickness of the electroluminescent layer, unevenness of multilayer of an electrode, or fine dust. In addition, due to malfunction of a transistor connected to an EL element, charges are applied in some cases to the EL element during a period when no charge is to be applied, thereby the EL element may emit light. On the contrary, due to malfunction of a transistor connected to an EL element, charges cannot be applied to the EL element in some cases, thereby the EL element would emit no light all the time. Thus, in some cases, an excess current may flow to an electroluminescent layer of an EL element or an EL element may emit light during a period when no light is to be emitted, which results in a significant degradation in quality of a display device. Note that in this specification, among a plurality of pixels each having a light emitting element, a pixel where a light emitting element always emits light or no light or a light emitting element cannot be controlled properly due to a short circuit between two electrodes of the light emitting element, a short circuit between wires, a short circuit between an electrode of the light emitting element and a wire, or malfunction of a transistor connected to the light emitting element, is referred to as a defective pixel.
When a defective pixel is included, a short circuit causes a voltage drop, and it is thus difficult in some cases to apply a sufficient potential to other pixels from a power supply line connected to each pixel. That is to say, a light emitting element that always emits light adversely affects light emitting elements in other pixels as well as the defective pixel.
The case here considered is a liquid crystal display device that has been growing in the display device market in recent years. In the case of a liquid crystal display device, a liquid crystal element operates as a capacitor and holds an applied voltage, and liquid crystal molecules are controlled by an electric field generated by the applied voltage. In other words, current does not keep flowing to a liquid crystal element. Accordingly, even if a pixel electrode of a pixel is short circuited to another wire, other pixels are not adversely affected as current does not keep flowing.
In addition, in the case of a liquid crystal element, power consumption is not increased since current does not keep flowing. Further, in general, only one transistor is provided in each pixel; therefore, one transistor and one pixel electrode are only required to be corrected.
On the other hand, an EL element is a current-driven device, where current keeps flowing from a power supply line during a light emitting period. When a defect occurs in a pixel provided with an EL element having such properties, other normal pixels may be adversely affected and power consumption may be increased. In addition, at least two transistors are provided in a pixel, and a circuit including a portion for holding a signal, a portion for controlling the amount of current and the like has a complex configuration. Thus, it is difficult to determine which portion is corrected and how the portion is corrected.
Such problems do not occur in a liquid crystal display device and are unique to an electroluminescence display device where a power supply line is required and current keeps flowing.
It is a primary object of the invention to provide a display device with improved image quality by correcting a defective pixel, in particular defects in a pixel where a light emitting element always emits light or a light emitting element cannot be controlled properly due to a short circuit between two electrodes of the light emitting element, a short circuit between wires, a short circuit between an electrode of the light emitting element and a wire, or malfunction of a transistor connected to the light emitting element. It is another object of the invention to provide a display device having a structure capable of easily correcting defective pixels.